A good question. The answer is that I couldn't remember exactly when Tony Blair first came to power, it was of course 1997. So a better example would be "Tony Blair has been Prime Minister since 1997." The example is "good" in that it illustrates the fact that the present perfect deals with past time from the point "now." It looks back in time from the present (hence the term present perfect) into the past and is therefore used when either the exact date of an event is not important or when a situation may still be true. The following hopefully illustrate these points.

I went to America in 1987. (past simple)

I have been to America once. (present perfect)

I lived in Beijing for two years. (past simple)

I have lived in Beijing for two years. (present perfect)

The past simple in contrast deals with events which have ended e.g. I do not live in Beijing now, or with exact dates of events e.g. 1997.

Kevin Vosper wrote:So a better example would be "Tony Blair has been Prime Minister since 1987." The example is "good" in that it illustrates the fact that the present perfect deals with past time from the point "now." It looks back in time from the present (hence the term present perfect) into the past and is therefore used when either the exact date of an event is not important or when a situation may still be true.

Thank you for your explanation repeating what I have known of. "Since 1987" does illustrate exactly what you have described here. But my question is, what kind of time is "Since 1987", as now he is still the PM? If you want to dwell on the twilight zone of Blair's departure, I would like to go back to my own example below.

What about "I have worked here since 2000"? That is, I am still working here. What kind of time is "Since 2000"? Is it past, present, or future?

Probably the best way to analyse your sentence is to take the linguistic approach and divide what most teachers call "tense" into its three basic parts. e.g tense, aspect and mood.

Tense refers to the way verb endings change in order to show differences in time for an event. In English there are two tenses, past and present.

Aspect refers to how we see the event e.g. as a whole, as an action of limited duration or as a view backwards from a certain point in time. In English this gives us the simple, the continious and the perfect.

Moods gives a psychological dimension to the verb phrase and usually, though not always, uses the modal verbs.

Refering back to your sentence what is important is the perfect aspect of the sentence in that it gives a certain perception of the event from the point "now." The question is it past, present or future time is really meaningless as it is dealing with a perception of time not a simple fact. As I've said before, the best description is to say that it looks back into the past from the present. It may suggest continuality into the future e.g. I'm still working here and will be next week, but this is not stated by the verb form itself.

The final form of an English verb is a combination of these three basic parts, each bringing a certain part of the meaning to the verb as a whole.

Kevin Vosper wrote:Tense refers to the way verb endings change in order to show differences in time for an event. In English there are two tenses, past and present.

I am aware of this ABC, and I agree with you 100 percent. Now "I have worked here since 2000" obviously talks about time, as you will not deny. The question is what kind of time it is – past or present?

odyssey wrote:If you are the host of englishtense.com, why do you have to ask these questions?

What questions? Do you mean we should not ask simple questions here in the discussion forum? Any time before now is past. Even the past minute or the past second is past time, so "the past three months" must be a past time. It is as simple as can be.

On the other hand, your question strikes me odd. Please tell me, are there some kinds of questions we are not allowed to discuss here?

odyssey wrote:Or put another way, if you have to ask these questions, why are you the host of englishtense.com?

Didn't you see my reason at the beginning?
"I am studying the nomenclature of Since." As people agree that tense is used to express time, I am trying to learn more about how to call a time past, present, or future.

On the other hand, may you explain why I shouldn't ask questions, just because I am the host of englishtense.com?

Any time before now is past. Even the past second or the past minute is past time. Therefore, it is beyond doubt that "the past three months" is a past time. With the adjective 'past', doesn't the time adverbial declare itself clear enough that it is a past time? But the problem is, it can stay with Present Perfect tense:
Ex: He has worked there in the past three months.
If the example is a correct structure, which it is, it must be adding a footnote to the common idea that Present Perfect cannot stay with past time adverbial:
Ex: *He has worked there last month.

As is pointed out in the following web page:
http://www.englishtense.com/newapproach/3_1.htm#3_1_10though past time cannot come up and mix with present time, a past action can. The chance is always there that an action starting in a certain past time may not end by now. In this case, we use the pattern of Since 2000 or in the past three months to say it:
Ex: He has worked there since 2000/in the past three months.
In this case, it may be said that Present Perfect is also compatible with past time adverbials.

In the old way of explaining tense, since people have been clinching on one-sentence basis (using one sentence and one tense in illustrating the use of a tense), they have long confused tense and sentence as one and the same. They can no longer separate action from time. Now as they see the time is a past, they reason the action must be also past:
Ex: He has worked there in the past three months.
== However, this time, the action is not a past action.
Grammarians are deeply puzzled. Believe it or not, they hide the pattern of in the past three months away from their books. The concealment has now become a must, as is pointed out in the following page:
http://www.englishtense.com/newapproach/2_4.htm== I call the pattern of in the past three months the Past Family.
I have talked about these past time adverbials for decades, informing numerous readers and webmasters, and yet today there is still not a grammar book or web page that mentions about them.

What has happened here? The patterns of in the past three months and since 2000 are used with Present Perfect, rather than Simple Past. They play an important role in rendering Present Perfect. But they disappear when they should have been there. Now you may see what I mean when I say at the front page of my website that old grammars fail to explain the Present Perfect tense. Can we explain a tense by avoiding the difficult yet important part of the tense?

I am not really showing a difficulty here. On the contrary, if we put sentences and different tenses together, we will easily see a time sequence of past-perfect-present, with the Perfect Time in between past and present. And the sole function of Present Perfect is to express the Perfect Time. Therefore, if we will, we can replace huge difficulties and atrocious concealments with an absolute simplicity.

In my humble website, our new approach to the three tenses has arrived at a simplicity:
-- Simple Past expresses past time-- Present Perfect expresses perfect time-- Simple Present expresses present time"One more word to it is a mistake" — this is my promise.
We can defend such simplicity because our new approach preaches that let the sentence be the sentence, and let the tense be the tense. Once the sentence-tense confusion has cleared up, the tense is a simplicity.

it's an example of what i was talking about in the other topic thread. it doesn't matter. both are correct answers; Worked and Have Been Working, when having a conversation with someone. we use both answers.